The processing of crude petroleum gives rise to various hydrocarbon fractions which may be subsequently "cracked" by heating, usually in the presence of steam to produce a range of lower boiling products.
U.S. Pat. No. 2,734,046 teaches that light colored resins can be produced from unsaturated petroleum fractions obtained by steam cracking when the fraction is substantially free of cyclic dienes by the use of an aluminum halide catalyst. Briefly, the broad concept of the teaching includes the following steps.
Petroleum fractions such as kerosene, gas oil, naphtha, etc. are cracked in the presence of steam at temperatures of about 530.degree. C. to 815.degree. C. to give highly unsaturated products. The liquid cut boiling largely below about C.sub.9 is segregated and heated at about 90.degree. to 140.degree. C. to dimerize cyclopentadienes. Thereafter a C.sub.8 to C.sub.9 and lighter liquid cut including C.sub.5 is taken overhead and there is separated a dimer concentrate as bottoms. The overhead stream, thus largely freed of cyclodienes to a level of about 2 weight percent is the raw material for making the resins. This stream is treated with an aluminum halide catalyst at about -40.degree. to 70.degree. C. under conditions of good agitation. The resin thus formed may be recovered by water and/or alkali washing to remove catalyst followed by stripping off the unpolymerized material as is more fully taught in U.S. Pat. No. 2,770,613.
It then developed that this overhead stream which was largely freed of cyclodienes was better controlled in composition for the preparation of these aliphatic resins by fractionating the overhead stream which contained more than 3.5 weight percent isoprene to reduce the isoprene content to below about 3.5 weight percent, recovering said fraction reduced in isoprene content and having the following distillation analysis and composition:
______________________________________ Weight Percent ______________________________________ Distillation I.B.P. -38.degree. C. 0-15 38-70.degree. C. 25-50 70-130.degree. C. 35-70 130.degree.+ &lt;5 Composition Diolefins, Conj. 11-25 Aromatics 18-41 Paraffins 0-5 Olefins 35-70 ______________________________________
and polymerizing said latter fraction in the presence of an aluminum halide catalyst at a temperature between -40.degree. to +70.degree. C. to produce a resinous product (see U.S. Pat. No. 2,894,937).
Although thhe resulting resins are of lighter color and have excellent softening points that were realized without the above-mentioned composition control step, the colors obtained are still too dark for many industrial applications provoked by technological developments in pressure sensitive adhesives for which petroleum resins modifying, e.g. tackifying, properties in the elastomer based formulations, and hot tack properties in the hot melt adhesive formulations based on ethylene polymers.
The literature also teaches the desirability of improving the color and thermal stability of aromatic hydrocarbon resins by reacting the resin feed stream with a dienophile, e.g. maleic anhydride, prior to polymerization (U.S. Pat. Nos. 4,105,843 and 4,230,840).
It is therefore an object of this invention to provide petroleum resins, particularly aliphatic, of much lighter color than presently available by an improved process for making said resins.